This invention relates to single crystal diamond.
Diamond offers a range of unique properties, including optical transmission, thermal conductivity, stiffness, wear resistance and its electronic properties. Whilst many of the mechanical properties of diamond can be realised in more than one type of diamond, other properties are very sensitive to the type of diamond used. For example, for the best electronic properties CVD single crystal diamond is important, often outperforming polycrystalline CVD diamond, HPHT diamond and natural diamond.
In many applications of diamond the limited lateral dimensions of the diamond available is a substantial limitation. Polycrystalline CVD diamond layers have substantially removed this problem for applications where the polycrystalline structure is suitable for the application, but in many applications polycrystalline diamond is unsuitable.
Whilst natural and HPHT diamond may not be suitable for some applications, they are used as substrates on which to grow CVD diamond. Although substrates can have a variety of crystallographic orientation, the largest and most suitable substrate orientation which can be produced for growth of high quality CVD diamond is generally (001). Throughout this specification, the Miller indices {hkl}, defining a plane based on the axes x,y,z will be written assuming that the z direction is that normal to the substrate surface and parallel to the growth direction. The axes x,y are then within the plane of the substrate, and are generally equivalent by symmetry but distinct from z because of the growth direction.
Large natural single crystal diamond is extremely rare and expensive, and large natural diamond substrate plates suitable for CVD diamond growth have not been demonstrated because of the associated very high economic risk in their fabrication and use. Natural diamond is often strained and defective, particularly so in larger substrate plates, and this causes twins and other problems in the CVD overgrowth or fracture during synthesis. In addition, dislocations which are prevalent in the natural diamond substrate are replicated in the CVD layer, also degrading its electronic properties.
HPHT synthetic diamond is also limited in size, and generally is of poorer quality in the larger stones, with inclusions being a major problem. Larger plates fabricated from synthetic diamonds generally exhibit missing corners so that edge facets other than {100} (such as {110}) are present, or they are included or strained. During synthesis further facets are formed, such as the {111} which lies between the (001) top face and the {110} side facets (see FIG. 1 of the accompanying drawings). In recent years significant effort has been directed at synthesising HPHT diamond of high quality for applications such as monochromators, and some progress has been reported, but the size of HPHT plates suitable for substrates remains limited.
{111} faces in particular are known generally to form twins during CVD synthesis of thick layers, limiting the area of perfect single crystal growth and often leading to degradation and even fracture during synthesis, further exacerbated by thermal stresses resulting from the growth temperature. Twinning on the {111} particularly interferes with increasing the size of the largest plate which can be fabricated with a (001) major face and bounded by {100} side faces.
Routinely available (001) substrates range up to about 7 mm square when bounded by {100} edges, and up to about 8.5 mm across the major face when bounded by {100} and {110} edges.
CVD homoepitaxial synthesis of diamond involves growing CVD epitaxially on an existing diamond plate and is well described in the literature. This is of course still limited by the availability of existing diamond plates. In order to achieve larger areas, the focus has been to grow laterally as well, increasing the overall area of the overgrown plate. Such a method is described in EP 0 879 904.
An alternative to homoepitaxial growth is heteroepitaxial growth, where a non-diamond substrate is grown on with an epitaxial relationship. In all reported cases however, the product of this process is quite distinct from homoepitaxial growth, with low angle boundaries between highly oriented but not exactly oriented domains. These boundaries severely degrade the properties of the diamond.
Homoepitaxial diamond growth to enlarge the area of a CVD plate presents many difficulties.
If it was possible to achieve ideal homoepitaxial growth on a diamond plate, the growth which would be achieved is substantially that illustrated by FIGS. 1 and 2 of the accompanying drawings. The growth morphology illustrated assumes that there is no competing polycrystalline diamond growth. However, in reality, there is generally competition from polycrystalline growth, growing up from the surface on which the diamond substrate plate is mounted. This is illustrated by FIG. 3 of the accompanying drawings.
Referring to FIG. 3, a diamond substrate plate 10 is provided mounted on a surface 12. Example materials for surface 12 include molybdenum, tungsten, silicon and silicon carbide. During CVD diamond growth, single crystal diamond growth will occur on the (001) face 14 and on the side surfaces, two of which 16 are shown. The side surfaces 16 are {010} surfaces. Growth will also occur on and extend outwards from the corners and vertices 18 of the plate. All such growth will be homoepitaxial single crystal growth. The growth on each of the faces present on the substrate, and on any new surfaces generated during growth, constitutes a growth sector. For example, in FIG. 3 diamond growth 24 arises from the {101} plane and thus is the {101} growth sector.
Competing with the homoepitaxial single crystal growth will be polycrystalline diamond growth 20 which will take place on the surface 12. Depending on the thickness of the single crystal diamond layer produced on the surface 14, the polycrystalline diamond growth 20 may well meet the homoepitaxial single crystal diamond growth along line 22, as illustrated in FIG. 3.
Based on FIG. 2, one might expect that the purely lateral growth on the substrate side surfaces could be used to fabricate a larger substrate, including the material of the original substrate. However, as is clear from FIG. 3, such a plate would actually contain competing polycrystalline growth. A plate fabricated parallel to the original substrate, but higher up in the grown layer is likely to contain twinning, especially from material in the {111} growth sector.
Under growth conditions where polycrystalline diamond does not compete with the single crystal diamond there still remains the problem that the quality of the lateral single crystal growth is generally poor, as a result of the different geometry and process conditions present at the diamond substrate edges, exacerbated by the method used to suppress polycrystalline growth.
Defects in the substrate used for CVD diamond growth replicate into the layer grown thereon. Clearly, since the process is homoepitaxial, regions such as twins are continued in the new growth. In addition, structures such as dislocations are continued, since by its very nature a line dislocation cannot simply self terminate, and the probability of two opposite dislocations annihilating is very small. Each time a growth process is initiated, additional dislocations are formed, primarily at heterogeneities on the surface, which may be etch pits, dust particles, growth sector boundaries and the like. Dislocations are thus a particular problem in single crystal CVD diamond substrates, and in a series of growths in which the overgrowth from one process is used as the substrate for the next, the density of dislocations tends to increase substantially.